A wavelength-division mutliplex (WDM) long haul optical network typically consists of a number of co-located transmitters (typically electrical-to-optical converters), a multiplexer to combine multiple wavelengths of light onto the same fiber, spans of buried or aerial plant fiber (each typically 40 to 120 km long), optical amplifier sites at the end of each span, a demultiplexer to separate out the wavelengths, and a set of receivers (typically optical-to-electrical converters).
One property of optical fiber which needs to be managed for high bit rate long haul optical networks is dispersion. Dispersion is the tendency for different optical wavelength components of an optical signal to propagate at different speeds, leading to distortion at the receivers. Typically, dispersion is compensated by placing a component, known as a dispersion and slope compensation module (DSCM), in some or all of the optical amplifier sites as well as multiplexer and demultiplexer sites. The DSCM has dispersion characteristics of opposite sign to the plant fiber. The net dispersion at any point is the sum of all fiber span and DSCM dispersions to that point, hence dispersion can be managed by choosing each DSCM to have the appropriate dispersion characteristics to cancel plant fiber dispersion and meet specified net dispersion targets at each optical amplifier site, as well as the receiver site.
A DSCM typically consists of a spool of optical fiber with fixed characteristics, placed in a circuit pack which goes into a shelf at an optical amplifier site. Because conventional technology is static, i.e., the dispersion of each DSCM is fixed, a problem arises when the dispersion of the optical plant fiber in the ground (or in the air) changes due to daily or seasonal temperature variations. Specifically, each type of fiber has a temperature coefficient (in ps/nm/km/° C.) which linearly relates the change in dispersion per unit path length to a change in temperature. Thus, when the temperature of a span changes, the dispersion of that span changes in proportion therewith. However, because the dispersion of each DSCM is fixed, the result is a drift of the net dispersion across each span as well as at the receiver. This problem can be referred to as temperature-induced dispersion drift.
Typically, the problem of temperature-induced dispersion drift is dealt with at the design phase by factoring a margin into the optical link budget. This forces the installation of optical amplifiers at shorter inter-amplifier distances in order to guarantee the integrity of the information being transmitted over the optical link, across an expected temperature range. Otherwise, if the optical amplifiers are kept far apart, there is an increased likelihood that the signals being received at the receiver will be distorted due to temperature-induced dispersion drift. The problem is further exacerbated as bit rates increase to 40 Gbps and beyond, and/or as link lengths increase. Thus, the need to actively compensate for temperature-induced dispersion drift in optical communication links becomes apparent.